Injury to living tissue results from the transfer of energy to atoms and molecules
in the cellular structure. Ionizing radiation causes atoms and molecules to become
ionized or excited. These excitations and ionizations can:

The cell can repair certain levels of cell damage. At low doses, such as that received
every day from background radiation, cellular damage is rapidly repaired.

At higher levels, cell death results. At extremely high doses, cells cannot be replaced
quickly enough, and tissues fail to function.

Tissue Sensitivity

In general, the radiation sensitivity of a tissue is:

proportional to the rate of proliferation of its cells

inversely proportional to the degree of cell differentiation

For example, the following tissues and organs are listed from most radiosensitive
to least radiosensitive:

Most Sensitive: Blood-forming organs

Reproductive organs

Skin

Bone and teeth

Muscle

Least sensitive: Nervous system

This also means that a developing embryo is most sensitive to radiation during the
early stages of differentiation, and an embryo/fetus is more sensitive to radiation
exposure in the first trimester than in later trimesters.

Prompt and Delayed Effects

Radiation effects can be categorized by when they appear.

Prompt effects: effects, including radiation sickness and radiation burns, seen immediately after
large doses of radiation delivered over short periods of time.

Delayed effects: effects such as cataract formation and cancer induction that may appear months or
years after a radiation exposure.

Prompt Effects

High doses delivered to the whole body of healthy adults within short periods of time
can produce effects such as blood component changes, fatigue, diarrhea, nausea and
death. These effects will develop within hours, days or weeks, depending on the size
of the dose. The larger the dose, the sooner a given effect will occur.

* The LD 50/60 is that dose at which 50%of the exposed population will die within
60 days.

Partial Body Effects

These acute effects apply only when the whole body is relatively uniformly irradiated.
The effects can be significantly different when only portions of the body or an individual
organ system are irradiated, such as might occur during the use of radiation for medical
treatment. For example, a dose of 500 rem delivered uniformly to the whole body may
cause death while a dose of 500 rem delivered to the skin will only cause hair loss
and skin reddening.

Delayed Effects of Radiation Exposure

Cataracts

Cataracts are induced when a dose exceeding approximately 200-300 rem is delivered
to the lens of the eye. Radiation-induced cataracts may take many months to years
to appear.

Cancer

Studies of people exposed to high doses of radiation have shown that there is a risk
of cancer induction associated with high doses.

There may be a risk of cancer at low doses as well. The following frames discuss the
risk of cancer at lower doses.

Process of Determining Cancer Risk

Why are cancer risks at low doses uncertain?

It has been difficult to estimate cancer induction risks, because most of the radiation
exposures that humans receive are very close to background levels. At low dose levels
of millirems to tens of rems, the risk of radiation-induced cancers is so low, that
if the risk exists, it is not readily distinguishable from normal levels of cancer occurrence.
In addition, leukemia or solid tumors induced by radiation are indistinguishable from
those that result from other causes.

Cancer Risk Estimates

Using the linear no-threshold risk model, the 1990 BEIR* V report provided the following
estimate:

The average lifetime risk of death from cancer following an acute dose equivalent
to all body organs of 0.1 Sv (10 rem) is estimated to be 0.8%. This increase in lifetime
risk is about 4% of the current baseline risk of death due to cancer in the United
States . The current baseline risk of cancer induction in the United States is approximately
25%.

Another way of stating this risk:

A dose of 10 mrem creates a risk of death from cancer of approximately 1 in 1,000,000.

* The National Academy of Sciences Committee on the Biological Effects of Ionizing
Radiation (the BEIR Committee)

Putting Risk into Perspective

One way of considering the level of a risk is to look at the number of "days lost"
out of a population due to early death from a given cause, then distributing those
days lost over the population to get an "average life expectancy lost" due to that
cause. The following table provides an estimate of life expectancy lost due to several
causes:

Health Risk

Estimated Life Expectancy Lost

Smoking 20 cigarettes a day

6 years

Overweight by 15%

2 years

Alcohol (US average)

1 year

all accidents

207 days

All natural hazards

7 days

Occupational dose of 300 mrem/year

15 days

Source: these estimates are taken from NRC Draft Guide DG-8012 and were adapted from
B. L. Cohen and L. S. Lee, "Catalogue of Risks Extended and Updates," Health Physics , Vol. 61, September 1991.

You can also look at risk by considering the Relative Risk of a 1 in a million chance of death from activities common to our society:

· Smoking 1.4 cigarettes in a lifetime (lung cancer)

· Eating 40 tablespoons of peanut butter (aflatoxin)

· Spending two days in New York City (air pollution)

· Driving 40 miles in a car (accident)

· Flying 2500 miles in a jet (accident)

· Canoeing for 6 minutes (drowning)

· Receiving a dose of 10 mrem of radiation (cancer)

Genetic Effects

There is no direct evidence of radiation-induced genetic effects in humans, even at
high doses. Various analyses indicate that the rate of genetic disorders produced
in humans is expected to be extremely low, on the order of a few disorders per million
live born per rem of parental exposure.

Prenatal Radiation Exposure

Rapidly proliferating and differentiating tissues are most sensitive to radiation
damage. Consequently, radiation exposure can produce developmental problems, particularly
in the developing brain, when an embryo/fetus is exposed prenatally.

The developmental conditions most commonly associated with prenatal radiation exposure
include low birth weight, microcephaly, mental retardation, and other neurological
problems. These effects are related to the developmental stage at which the exposure
occurs. The threshold dose for developmental effects is approximately 10 rem.

The evidence that the developing embryo/fetus is more sensitive to radiation-induced
cancer is inconclusive. But it is prudent to assume that there is some increased sensitivity.

This is the end of the Biological Effects Module, which is the third of the six Radiation
Basics modules. The next module is the Regulations Module.